1. Field
The present invention is concerned with a novel process for the manufacture of polyene (di)aldehydes from acetalised short-chain polyene (di)aldehydes by an acid-catalyzed condensation reaction with alkoxydienes.
2. Description
Lewis acid-catalyzed additions of .alpha.,.beta.-unsaturated ethers (enol ethers) to acetals have been known for a long time and date back to the work of Muller-Cunradi and Pieroh (see U.S. Pat. No. 2,165,962). Hoaglin and Hirsch J.A.C.S., 71: 3468 (1949)! investigated this reaction further and broadened the possible applications, which Isler et al. likewise did in the 1950's, namely with respect to the synthesis of .beta.-carotene, crocetin dialdehyde, lycopene as well as .beta.-apocarotenoids see Helv. Chim. Acta, 39: 249 et seq. and 463 et seq. (1956), ibid. 42: 854 et seq. (1959) as well as U.S. Pat. Nos. 2,827,481 and 2,827,482!. Later, Mukaiyama Angew. Chem., 89: 858 et seq. (1977) and Org. Reactions, 28: 203 et seq. (1982)! extended the reaction by using the advantageous and readily accessible trimethylsilyl enol ethers.
The first Lewis acid-catalyzed condensations of 1-alkoxy-1,3-dienes (dienol ethers) with .alpha.,.beta.-unsaturated acetals were reported by Nazarov and Krasnaya J. Gen. Chem. USSR, 28: 2477 et seq. (1958)! and by Makin Pure & Appl. Chem., 47: 173 et seq. (1976), J. Gen. Chem. USSR, 31: 3096 et seq. (1961) and 32: 3112 et seq. (1962)!. Here, the coupling of the acetal to the dienol ether takes place as far as can be seen exclusively at its .gamma.-position with the formation of a chain-lengthened (.alpha.,.beta.-unsaturated acetal, which, however, in competition with the first acetal reacts with further dienol ether with the formation of a further, chain-lengthened .alpha.,.beta.-unsaturated acetal etc. telomer formation; see also Chemla et al., Bull. Soc. Chim. Fr., 130: 200 et seq. (1993)!. For this reason such a condensation has been found not to be workable for synthetic purposes, especially for the synthesis of apocarotenoids Isler et al., Adv. Org. Chem., 4: 115 et seq. (1963)!.
Not only 1-alkoxy-1,3-dienes, but also trimethylsilyloxydienes of the type CH.sub.2 .dbd.CH--CH.dbd.CH--OSi(CH.sub.3).sub.3 ! can be condensted with .alpha.,.beta.-unsaturated acetals in the presence of Lewis acid catalysts, as disclosed by Mukaiyama et al. in Chem. Lett. 1975, 319 et seq. In this coupling too the attack seems to take place exclusively at the terminal (.gamma.-) carbon atom of the diene system to form ".gamma.-products" (see Mukaiyama et al., Bull. Chem. Soc. Jap 50: 1161 et seq. (1977) and Japanese Patent Publication (Kokai) 36,645/1977!. In contrast to the reaction with 1-alkoxy-1,3-dienes, from which an (.alpha.,.beta.-unsaturated acetal is produced, the reaction of trimethylsilyloxydienes with acetals affords an aldehyde which cannot react further with the diene (no telomer formation). By using this method Mukaiyama et al. were able to synthesize vitamin A see Kokai 36,645/1977, Chem. Lett. 1975, 1201 et seq. and Bull. Chem. Soc. Japan 51: 2077 et seq. (1978)! and workers from Rhone-Poulenc developed new routes to carotenoids and vitamin A (see DOS 2,701,489 and A.E.C. Societe de Chimie Organique et Biologique No. 7824350).
The aforementioned Lewis acid-catalyzed condensation of a dienol ether with an .alpha.,.beta.-unsaturated acetal based on the works of Nazarov and Krasnaya, Makin as well as Chemla et al. would be a very valuable access to apocarotenals and bis-apocarotenals if the yields of the desired primary product of the type . . . CH.dbd.CH--CH(Oalkyl.sup.1)--CH.sub.2 --CH.dbd.CH--CH(Oalkyl.sup.1)(Oalkyl.sup.2) could be increased and the telomer formation could be suppressed. Thus, the desired polyene aldehyde of the type . . . CH.dbd.CH--CH.dbd.CH--CH.dbd.CH--CHO could be obtained from this primary product by hydrolysis of the acetal group C(Oalkyl.sup.1)(Oalkyl.sup.2) and elimination of alkyl.sup.1 OH. In addition to the fact that in this reaction the formation of the double bond takes place under catalytic conditions, no phosphorus-, silicon- or sulphur-containing reagents are required.
The object of the present invention is to manufacture chain-lengthened polyene aldehydes or dialdehydes starting from polyene acetals or diacetals while avoiding as far as possible the aforementioned disadvantages of the state of the art and replacing the Wittig, Horner or Julia reaction hitherto used for this purpose.